1. Field of the Invention
The present invention relates to a printed circuit board having embedded capacitors therein and a manufacturing process thereof. More particularly, the present invention relates to a process for manufacturing a printed circuit board having embedded capacitors therein, in which dielectric layers are formed using a ceramic material having high dielectric capacitance by atomic layer deposition (ALD), so as to form a capacitor having a high dielectric constant corresponding to the capacitance of decoupling chip capacitors, as well as a printed circuit board manufactured by the method.
2. Description of the Prior Art
Common discrete chip resistors or discrete chip capacitors have been mounted on the surfaces of most printed circuit boards (PCBs). Recently, printed circuit boards having embedded passive devices, such as resistors, capacitors, etc., have been developed.
Such printed circuit boards having embedded passive devices therein are manufactured by a process comprising inserting passive devices such as resistors, capacitors, etc., into an inner or outer layer of a substrate using novel materials (substances) and processes, thereby replacing the functions of conventional chip resistors and chip capacitors. That is, the printed circuit boards having embedded passive devices therein include passive devices, e.g., capacitors, buried in the inner or outer layer of the substrate. The term “embedded capacitors” as used herein refers to capacitors as passive devices which are mounted as parts of a printed circuit board, regardless of the size of a substrate. The substrate is referred to as an “embedded capacitor PCB”. The most important advantage of the embedded capacitor PCB is that since the capacitors are integrated as parts of the printed circuit board, there is no need to mount the capacitors on the surfaces of the printed circuit board.
Meanwhile, recent techniques for manufacturing the capacitor-embedded printed circuit board are largely classified into the following three techniques:
The first technique is a method for manufacturing polymer thick film type capacitors comprising applying a polymer capacitor paste and thermal curing (that is, drying) the paste. Specifically, according to this method, the embedded capacitors are formed by applying the polymer capacitor paste on the printed circuit board inner layer, drying the polymer capacitor paste, printing a copper paste on the resulting printed circuit board to form electrodes and drying the copper paste.
The second technique is a method for manufacturing embedded discrete type capacitors comprising coating a ceramic filled photo-dielectric resin on a printed circuit board. The related patent is held by Motorola Inc. (see U.S. Pat. No. 6,349,456). Specifically, according to this method, the discrete capacitors are formed by applying the photo-dielectric resin containing ceramic powder on both surfaces of a substrate, laminating copper foils thereto to form respective top electrodes and bottom electrodes, forming circuit patterns thereon, and etching the photo-dielectric resin.
The third technique is a method for manufacturing capacitors comprising inserting dielectric layers having a capacitance characteristic into the printed circuit board inner layer, thereby replacing decoupling capacitors which have been mounted on the surfaces of the conventional printed circuit boards. The related patent is owned by Sanmina Corp. (see U.S. Pat. Nos. 5,079,069, 5,261,153, and 5,800,575). Specifically, according to this method, power-distributed decoupling capacitors are formed by a process comprising inserting dielectric layers composed of power electrodes and ground electrodes into the inner layers of the printed circuit board.
A variety of processes have been developed based on the three techniques discussed above. Procedures for carrying out the processes are different from each other. Since the market for printed circuit boards having embedded capacitors therein is small, these techniques have not yet been standardized and trials to develop commercially available processes are still ongoing.
The above-described embedded capacitors are advantageous in that, because the capacitors are inserted into a board, they can reduce the area occupied by chip capacitors, leading to an increase in the mounting density of chips. Another advantage is that the embedded capacitors eliminate the need to mount chip capacitors on the surface. In the prior art, the connection distance between devices is so long that electrical parasitism occurs at a high level, thereby reducing the electric performance of products. Also, an increase in the number of electrical connections by soldering, etc., causes a problem with the reliability of products. However, the use of embedded capacitors can shorten the connection length between devices to reduce electrical parasitism, thus improving the electrical performance of products.
The material of the embedded capacitors in the embedded capacitor PCBs according to the prior art is a polymer material or ceramic-filled photosensitive resin which is suitable for use in printed circuit board processes. However, there is a problem in that the material is too low in dielectric capacitance to perform the role of chip capacitors.
To obtain high-capacity capacitors, a dielectric material having a high dielectric constant needs to be used. The smaller the thickness of the dielectric material and the larger the surface area, the higher the capacitance of capacitors.
For example, U.S. Pat. No. 6,274,224 issued to 3M Corporation discloses an embedded capacitor fabricated by forming a thick film-type dielectric layer having a thickness of 0.5-10 μm between copper foils acting as power and ground electrodes, the capacitor being formed of a composite of BaTiO3 ceramic powder and thermosetting resin, such as epoxy or polyimide. The embedded capacitor according to said patent has the following properties: surface roughness of the copper foils: 10-300 nm; capacitance per unit area of the dielectric layer: 10 nF/in2 or more; and adhesion: 3 lb/inch or more. As described above, the capacitance per unit area of the embedded capacitors according to the prior art is about 5-1 nF/in2 or 10 nF/in2, which is significantly lower than 100 nF/in2 for generally used decoupling discrete chip capacitors. This indicates that there are many limitations in forming embedded capacitors having high capacitance.
High dielectric materials and process conditions used in forming embedded capacitors according to the prior art are summarized in Table 1 below.
TABLE 1FormationThicknesstemperatureDielectricDielectricof dielectricof dielectriccapacitancematerialsthin film (μm)thin film (° C.)(nF/in2)Methods of forming embedded capacitorsTa2O50.3500425Forming a Ta2O5 film 0.3 μm thick on a Si substrate(50 μm thick) by an anodic oxidation process,forming a top electrode (Cr/Cu) thereon andfabricating multi-layer PCB through a build-upprocess.BaTiO310350650Forming a BaTiO3 ceramic thin film 0.5 μm thick ona copper foil by a sol-gel process, and forming a topelectrode (Ni/Cu) by thermal treatment at 350° C.SrTiO30.6>500300Forming a bottom electrode (0.2 μm thick) in theFR4 substrate, depositing an SrTiO3 film 0.3 μmthick thereon by sputtering, and forming a topelectrode (Ni/Cu) on the film.(Ba, Sr)TiO30.12602400Forming dielectric films of 100 nm on a Si substrateby sputtering at 260° C., and forming Pt films as topand top electrodes. thereonCeramic film3R.T.2580Forming a dielectric thin film on the FR4 substrate(Dk = 400)by an aerosol deposition process.BaTiO325900 15-700Forming a ceramic paste on a copper foil by screenprinting, sintering the paste at 900° C. in a nitrogenatmosphere, and forming a top electrode using aconductive paste.(Pb, Zr)TiO30.26501290-1935Forming a (Pb, Zr)TiO3 film on a copper foil by asol-gel process, crystallizing the film at 650° C. in anitrogen atmosphere, and forming top and bottomelectrodes using Cu/Ni.
As shown in Table 1, a dielectric ceramic material having a high dielectric constant is difficult to actually apply onto a printed circuit board because it necessarily involves a crystallization process at a high temperature, at which deformation of the resin board occurs. Also, if the dielectric material can be formed into a film at low temperature, the material will be difficult to apply onto a printed circuit board having a large area.
Accordingly, there is an urgent need for technology to manufacture a printed circuit board including embedded capacitors which not only have a higher dielectric capacitance than sheet-type capacitors embedded in the prior printed circuit boards, but also can be formed to have a large area at relatively low process temperatures.